The first studies to scan the entire human genome for genetic variations associated with heart disease didn’t find what researchers expected. More than two-thirds of the newly identified risk variants had no relationship to known risk factors for heart diseases such as cholesterol, high blood pressure or diabetes.
As more recent studies have found further relevant genetic variants also lacking association with what we know about heart disease, researchers have been trying to unravel how these new risk factors help drive the initiation and progress of disease.
A recent landmark study now provides the strongest evidence to date that inflammation plays an important role in heart disease and that many of the newly identified genetic risk factors contribute to this process. The study was published this past December in Nature Genetics by the international CARDIoGRAMplusC4D consortium, which includes researchers at the University of Ottawa Heart Institute.
“Essentially, coronary artery disease is a manifestation of a normal process gone a little bit haywire,” explained Alexandre Stewart, PhD, Principal Investigator in the Heart Institute’s Ruddy Canadian Cardiovascular Genetics Centre and a co-author of the new study. “Cholesterol has to get through your blood vessels to tissues. Tissues need cholesterol to maintain cell membranes. But the problem is that too much of it ends up accumulating on the vessel walls, and that becomes inflammatory.”
The question has been why this process leads quickly to coronary artery disease in some people but not others. Around 85 per cent of people older than 50 years of age have plaques on their blood vessel walls caused by cholesterol accumulation, but only a minority will develop symptoms or suffer a heart attack.
The new paper suggests that the answer comes down to how a person’s genetic profile shapes his or her inflammatory response. In individuals with genetic variations that produce less sensitive immune systems, the inflammatory response to the initial buildup of cholesterol in the blood vessels would be relatively mild. But in individuals with highly sensitive immune systems, a strong inflammatory response to that initial blood vessel damage could perversely end up causing greater damage, leading to more buildup and accelerating the development of disease.
“I think that five years from now, this is the paper people will be quoting as evidence of a link between inflammation and atherosclerosis”
– Dr. Robert Roberts, President and CEO, UOHI
“Inflammatory responses are good if you want to fight off disease. If you have strong inflammatory mechanisms, you’ll fight infection very aggressively. But the collateral damage,” said Stewart, “is that in other processes that are inflammatory”—like the response to cholesterol in blood vessels—“you’re going to overamplify that mechanism.
“We finally realize that a lot of these genes we’ve discovered are mediating their effects by ‘raising the volume,’ if you will, on the inflammatory response,” he added.
In order to perform the study, CARDIoGRAMplusC4D collected DNA from more than 60,000 patients with coronary artery disease and more than 130,000 people without the disease, all of European or South Asian descent. Like previous genome-wide association studies (GWAs) performed by the consortium, they used “gene chips” that allow scientists to look for thousands of gene variations in a sample simultaneously.
The results confirmed the significance of 31 genetic variants previously associated with coronary artery disease as well as identifying 15 new genetic risk factors. Samples from an additional group of around 3,600 patients with heart disease and 12,000 people without were used to validate the results.
But what makes this study different from previous large GWAs is what came next:The researchers used a mathematical technique called network analysis to determine which known molecular pathways these genetic risk factors could potentially influence, such as ones involved in cholesterol metabolism, cellular movement and the immune system.
The top three pathways identified as overlapping with the new genetic risk factors all included genes involved in cholesterol metabolism, an expected result. But three of the four top pathways also included genes involved in inflammation, including the rapid inflammatory response caused by tissue injury.
The cholesterol metabolism and inflammation pathways overlapped and intersected to potentially influence genes involved in both processes. For example, in the modelled network, immune cells called macrophages take up oxidized LDL cholesterol and are transformed into foam cells, which are the basis of the plaques found in coronary artery disease. In turn, these foam cells secrete substances called cytokines that encourage an additional inflammatory response.
Going back to the lab, the researchers examined the levels of expression of inflammation markers in tissue samples. “We found that a fair number of these genes were correlated with elevated inflammatory markers,” said Stewart. This suggests that the genetic variations are not just “guilty by association” by being located near inflammatory genes, but they are actually elevating the expression of some of those inflammatory genes, he continued.
“I think that five years from now, this is the paper people will be quoting as evidence of a link between inflammation and atherosclerosis,” said Dr. Robert Roberts, President and CEO of the Heart Institute, Director of the Ruddy Centre and study co-author.
Much more work remains to be done. As Stewart explained, additional studies need to confirm the gene expression results in patients, and researchers must dive deeper into understanding the specific inflammatory mechanisms being “turned up” by the genetic variations. “If we can tease that apart, there may be some master regulators or major control points that we could target so that people don’t have such aggressive immune responses to cholesterol buildup.”
The CARDIoGRAMplusC4D consortium also has other work to do. The group is currently performing a GWAS to better understand how smoking damages blood vessel, and has plans to study how epigenetic factors—structural elements of the chromosomes that help control gene expression—also contribute to the development of heart disease. “Individually our small samples aren’t sufficient to address those questions, but there’s really strength in numbers. There are so many variables and so many questions we can address with our combined data that we can’t address individually,” concluded Stewart.